This is an application for a Mentored Physician Scientist Development Award for Hina W. Chaudhry, M.D. who will be an assistant professor of clinical medicine as of July 1, 2000. She will spend 80 percent of her time in a program of scientific training under the guidance of Dr. Debra Wolgemuth, Professor of Genetics and Development. This training will be driven by a project whose theme is to investigate the role of cyclin A in cardiomyocyte development proliferation, and terminal differentiation. The first specific aim is based on a transgenic mouse model of constitutive cyclin A expression in the heart. This is based on the observation that cyclin A levels in the heart drop precipitously shortly after birth in mice, rats, and humans, coincident with terminal differentiation of the heart. The mouse model has been generated based on the hypothesis that constitutive expression will maintain proliferation through the later phases of development, the neonatal period, and adulthood. The phenotype of these mice will be characterized with respect to heart size, cardiomyocyte hyperplasia/hypertrophy, altered cardiac function, an increase in nuclear ploidy and DNA synthesis, alteration of expression of other cell cycle proteins, apoptosis, and an altered response to injury. The second specific aim seeks to characterize molecular events that occur downstream of cyclin A binding to its kinase partners, cdk2 and cdc2. This is based on the hypothesis that cardiomyocyte cell cycle exit is coordinated by a cascade of inhibition whereby down regulation of cyclin A results in hypophosphorylation of RB or an RB-hke protein, and that constitutive expression of cyclin A in the heart may result in the maintenance of a phosphorylated state of such a protein, thereby resulting in persistent transcription of genes responsible for DNA replication and cell division. The third specific aim seeks to understand the mechanism underlying the down regulation of cyclin A in the heart. Methylation of promoter sequences is a well-established epigenetic modification that can result in gene silencing. Sequences of the cyclin A promoter thought to be crucial for gene transcription will by assayed for methylation and a potential correlation with cell cycle exit will be explored. The pursuit of these specific aims will provide the critical focus during which time the PI will gain an intensive training experience, learning new techniques as well as the methods of scientific inquiry under the immediate supervision of the Sponsor, an internationally recognized expert in the field of cell cycle and developmental biology. In addition to daily interactions with the Sponsor and members of the immediate laboratory, the PI will participate in a series of lectures given by the faculty of Genetics and Development, and participate in seminars through the Department of Medicine. Additionally, the Sponsor holds a series of Work-in-Progress seminars, in which members of the laboratory present intermediate data and receive critical suggestions from other laboratory members and invited faculty. These intellectual resources, combined with the fiscal resources to support theproject which will be provided by the Chairman of Medicine, will enable the PI to develop into an independent investigator by the conclusion of the training period.